A number of procedures are presently available for the detection of specific nucleic acid molecules. These procedures typically depend on sequence-dependent hybridisation between the target nucleic acid and nucleic acid probes which may range in length from short oligonucleotides (20 bases or less) to sequences of many kilobases (kb).
The most widely used method for amplification of specific sequences from within a population of nucleic acid sequences is that of polymerase chain reaction (PCR) (Dieffenbach, C. and Dveksler, G. eds. PCR Primer: A Laboratory Manual. Cold Spring Harbor Press, Plainview N.Y.). In this amplification method, oligonucleotides, generally 20 to 30 nucleotides in length on complementary DNA strands and at either end of the region to be amplified, are used to prime DNA synthesis on denatured single-stranded DNA. Successive cycles of denaturation, primer hybridisation and DNA strand synthesis using thermostable DNA polymerases allows exponential amplification of the sequences between the primers. RNA sequences can be amplified by first copying using reverse transcriptase to produce a complementary DNA (cDNA) copy. Amplified DNA fragments can be detected by a variety of means including gel electrophoresis, hybridisation with labelled probes, use of tagged primers that allow subsequent identification (eg by an enzyme linked assay), and use of fluorescently-tagged primers that give rise to a signal upon hybridisation with the target DNA (eg Beacon and TaqMan systems).
As well as PCR, a variety of other techniques have been developed for detection and amplification of specific nucleotide sequences. One example is the ligase chain reaction (1991, Barany, F. et al., Proc. Natl. Acad. Sci. USA 88, 189-193).
Another example is isothermal amplification which was first described in 1992 (Walker G T, Little M C, Nadeau J G and Shank D. Isothermal in vitro amplification of DNA by a restriction enzyme/DNA polymerase system. PNAS 89: 392-396 (1992) and termed Strand Displacement Amplification (SDA). Since then, a number of other isothermal amplification technologies have been described including Transcription Mediated Amplification (TMA) and Nucleic Acid Sequence Based Amplification (NASBA) that use an RNA polymerase to copy RNA sequences but not corresponding genomic DNA (Guatelli J C, Whitfield K M, Kwoh D Y, Barringer K J, Richmann D D and Gingeras T R. Isothermal, in vitro amplification of nucleic acids by a multienzyme reaction modeled after retroviral replication. PNAS 87: 1874-1878 (1990): Kievits T, van Gemen B, van Strijp D, Schukkink R, Dircks M, Adriaanse H, Malek L, Sooknanan R, Lens P. NASBA isothermal enzymatic in vitro nucleic acid amplification optimized for the diagnosis of HIV-1 infection. J Virol Methods. 1991 December; 35(3):273-86).
Other DNA-based isothermal techniques include Rolling Circle Amplification (RCA) in which a DNA polymerase extends a primer directed to a circular template (Fire A and Xu SQ. Rolling replication of short circles. PNAS 92: 4641-4645 (1995), Ramification Amplification (RAM) that uses a circular probe for target detection (Zhang W, Cohenford M, Lentrichia B, Isenberg H D, Simson E, Li H, Yi J, Zhang D Y. Detection of Chlamydia trachomatis by isothermal ramification amplification method: a feasibility study. J Clin Microbiol. 2002 January; 40(1):128-32.) and more recently, Helicase-Dependent isothermal DNA amplification (HDA), that uses a helicase enzyme to unwind the DNA strands instead of heat (Vincent M, Xu Y, Kong H. Helicase-dependent isothermal DNA amplification. EMBO Rep. 2004 August; 5(8):795-800.)
Recently, isothermal methods of DNA amplification have been described (Walker G T, Little M C, Nadeau J G and Shank D. Isothermal in vitro amplification of DNA by a restriction enzyme/DNA polymerase system. PNAS 89: 392-396 (1992). Traditional amplification techniques rely on continuing cycles of denaturation and renaturation of the target molecules at each cycle of the amplification reaction. Heat treatment of DNA results in a certain degree of shearing of DNA molecules, thus when DNA is limiting such as in the isolation of DNA from a small number of cells from a developing blastocyst, or particularly in cases when the DNA is already in a fragmented form, such as in tissue sections, paraffin blocks and ancient DNA samples, this heating-cooling cycle could further damage the DNA and result in loss of amplification signals. Isothermal methods do not rely on the continuing denaturation of the template DNA to produce single stranded molecules to serve as templates from further amplification, but on enzymatic nicking of DNA molecules by specific restriction endonucleases at a constant temperature.
The technique termed Strand Displacement Amplification (SDA) relies on the ability of certain restriction enzymes to nick the unmodified strand of hemi-modified DNA and the ability of a 5′-3′ exonuclease-deficient polymerase to extend and displace the downstream strand. Exponential amplification is then achieved by coupling sense and antisense reactions in which strand displacement from the sense reaction serves as a template for the antisense reaction (Walker G T, Little M C, Nadeau J G and Shank D. Isothermal in vitro amplification of DNA by a restriction enzyme/DNA polymerase system. PNAS 89: 392-396 (1992). Such techniques have been used for the successful amplification of Mycobacterium tuberculosis (Walker G T, Little M C, Nadeau J G and Shank D. Isothermal in vitro amplification of DNA by a restriction enzyme/DNA polymerase system. PNAS 89: 392-396 (1992), HIV-1, Hepatitis C and HPV-16 Nuovo G. J., 2000), Chlamydia trachomatis (Spears P A, Linn P, Woodard D L and Walker G T. Simultaneous Strand Displacement Amplification and Fluorescence Polarization Detection of Chlamydia trachomatis. Anal. Biochem. 247: 130-137 (1997).
The use of SDA to date has depended on modified phosphorthioate nucleotides in order to produce a hemi-phosphorthioate DNA duplex that on the modified strand would be resistant to enzyme cleavage, resulting in enzymic nicking instead of digestion to drive the displacement reaction. Recently, however, several “nickase” enzyme have been engineered. These enzymes do not cut DNA in the traditional manner but produce a nick on one of the DNA strands. “Nickase” enzymes include N.Alw1 (Xu Y, Lunnen K D and Kong H. Engineering a nicking endonuclease N.Alw1 by domain swapping. PNAS 98: 12990-12995 (2001), N.BstNB1 (Morgan R D, Calvet C, Demeter M, Agra R, Kong H. Characterization of the specific DNA nicking activity of restriction endonuclease N.BstNBI. Biol. Chem. 2000 November; 381(11):1123-5.) and Mly1 (Besnier C E, Kong H. Converting MlyI endonuclease into a nicking enzyme by changing its oligomerization state. EMBO Rep. 2001 September; 2(9):782-6. Epub 2001 Aug. 23). The use of such enzymes would thus simplify the SDA procedure.
In addition, SDA has been improved by the use of a combination of a heat stable restriction enzyme (Ava1) and Heat stable Exo-polymerase (Bst polymerase). This combination has been shown to increase amplification efficiency of the reaction from a 108 fold amplification to 1010 fold amplification so that it is possible, using this technique, to the amplification of unique single copy molecules. The resultant amplification factor using the heat stable polymerase/enzyme combination is in the order of 109 (Milla M. A., Spears P. A., Pearson R. E. and Walker G. T. Use of the Restriction Enzyme Ava1 and Exo-Bst Polymerase in Strand Displacement Amplification Biotechniques 1997 24:392-396).
To date, all isothermal DNA amplification techniques require the initial double stranded template DNA molecule to be denatured prior to the initiation of amplification. In addition, amplification is only initiated once from each priming event.
For direct detection, the target nucleic acid is most commonly separated on the basis of size by gel electrophoresis and transferred to a solid support prior to hybridisation with a probe complementary to the target sequence (Southern and Northern blotting). The probe may be a natural nucleic acid or analogue such as peptide nucleic acid (PNA) or locked nucleic acid (LNA) or intercalating nucleic acid (INA). The probe may be directly labelled (eg with 32P) or an indirect detection procedure may be used. Indirect procedures usually rely on incorporation into the probe of a “tag” such as biotin or digoxigenin and the probe is then detected by means such as enzyme-linked substrate conversion or chemiluminescence.
Another method for direct detection of nucleic acid that has been used widely is “sandwich” hybridisation. In this method, a capture probe is coupled to a solid support and the target nucleic acid, in solution, is hybridised with the bound probe. Unbound target nucleic acid is washed away and the bound nucleic acid is detected using a second probe that hybridises to the target sequences. Detection may use direct or indirect methods as outlined above. Examples of such methods include the “branched DNA” signal detection system, an example that uses the sandwich hybridization principle (1991, Urdea, M. S., et al., Nucleic Acids Symp. Ser. 24, 197-200). A rapidly growing area that uses nucleic acid hybridisation for direct detection of nucleic acid sequences is that of DNA microarrays, (2002, Nature Genetics, 32, [Supplement]; 2004, Cope, L. M., et al., Bioinformatics, 20, 323-331; 2004, Kendall, S. L., et al., Trends in Microbiology, 12, 537-544). In this process, individual nucleic acid species, that may range from short oligonucleotides, (typically 25-mers in the Affymetrix system), to longer oligonucleotides, (typically 60-mers in the Applied Biosystems and Agilent platforms), to even longer sequences such as cDNA clones, are fixed to a solid support in a grid pattern or photolithographically synthesized on a solid support. A tagged or labelled nucleic acid population is then hybridised with the array and the level of hybridisation to each spot in the array quantified. Most commonly, radioactively- or fluorescently-labelled nucleic acids (eg cRNAs or cDNAs) are used for hybridisation, though other detection systems can be employed, such as chemiluminescence.
A rapidly growing area that uses nucleic acid hybridisation for direct detection of nucleic acid sequences is that of DNA micro-arrays (Young R A Biomedical discovery with DNA arrays. Cell 102: 9-15 (2000); Watson A New tools. A new breed of high tech detectives. Science 289:850-854 (2000)). In this process, individual nucleic acid species, that may range from oligonucleotides to longer sequences such as complementary DNA (cDNA) clones, are fixed to a solid support in a grid pattern. A tagged or labelled nucleic acid population is then hybridised with the array and the level of hybridisation with each spot in the array quantified. Most commonly, radioactively- or fluorescently-labelled nucleic acids (eg cDNAs) were used for hybridisation, though other detection systems were employed.
Traditional methods for the detection of microorganisms such as bacteria, yeasts and fungi and include culture of the microorganisms on selective nutrient media then classification of the microorganism based on size, shape, spore production, characters such as biochemical or enzymatic reactions and specific staining properties (such as the Gram stain) as seen under conventional light microscopy. Viral species have to be grown in specialised tissue or cells then classified based on their structure and size determined by electron microscopy. A major drawback of such techniques is that not all microorganisms will grow under conventional culture or cell conditions limiting the usefulness of such approaches. With bacteria, for example, such as Neisseria  meningitidis, Streptococcus pneumoniae and Haemophilus influenzae (which all cause meningitis and amongst which N. meningitidis causes both meningitis and fulminant meningococcaemia) all three species are difficult to culture. Blood culture bottles are routinely examined every day for up to seven days, and subculturing is required. H. influenzae requires special medium containing both nicotinamide adenine dinucleotide and haemin and growth on Chocolate Agar Plates. Blood cultures require trypticase soy broth or brain heart infusion and the addition of various additives such as sodium polyanetholesulphonate. For microorganisms such as Clostridium botulinum, which causes severe food poisoning and floppy baby syndrome, the identification of the toxin involves injection of food extracts or culture supernatants into mice and visualization of results after 2 days. In addition, culturing of the potential microorganism on special media takes a week. Staphylococcus aureus enterotoxin (a cause of food poisoning as well as skin infections, blood infections, pneumonia, osteomyelitis, arthritis and brain abscesses) is detected in minute amounts by selective absorption of the toxin via ion exchange resins or Reverse Passive Latex Agglutination using monoclonal antibodies. Its relative, S. epidermis, leads to blood infections and contaminates equipment and surfaces in hospitals and health care machines and appliances.
Non-viral microorganisms can also be classified based on their metabolic properties such as the production of specific amino acids or metabolites during fermentation reactions on substrates such as glucose, maltose or sucrose. Alternatively, microorganisms can be typed based on their sensitivity to antibiotics. Specific antibodies to cell surface antigens or excreted proteins such as toxins are also used to identify or type microorganisms. However, all the above methods rely on the culture of the microorganism prior to subsequent testing. Culture of microorganisms is expensive and time consuming and can also suffer from contamination or overgrowth by less fastidious microorganisms. The techniques are also relatively crude in that many tests must be done on the same sample in order to reach definitive diagnosis. Most microorganisms can not be readily grown in known media, and hence they fall below levels of detection when a typical mixed population of different species of microorganism is present in the wild or in association with higher organisms.
Other methods for the detection and identification of pathogenic microorganisms are based on the serological approach in which antibodies are produced in response to infection with the microorganism. Meningococci, for example, are classifiable on the basis of the structural differences in their capsular polysaccharides. These have different antigenicities, allowing five major serogroups to be determined, (A, B, C, Y and W-135). Enzyme Linked Immunosorbent Assays (ELISA) or Radio Immuno Assay (RIA) can assess the production of such antibodies. Both these methods detect the presence of specific antibodies produced by the host animal during the course of infection. These methods suffer the drawback in that it takes some time for an antibody to be produced by the host animal, thus very early infections are often missed. In addition, the use of such assays cannot reliably differentiate between past and active infection.
More recently, there has been much interest in the use of molecular methods for the diagnosis of infectious disease. These methods offer sensitive and specific detection of pathogenic microorganisms. Examples of such methods include the “branched DNA” signal detection system. This method is an example that uses the sandwich hybridization principle (Urdea M S et al. Branched DNA amplification multimers for the sensitive, direct detection of human HIV and hepatitis viruses. Nucleic Acids Symp Ser. 1991; (24):197-200).
Another method for the detection and classification of bacteria is the amplification of 16S ribosomal RNA sequences. 16S rRNA has been reported to be a suitable target for use in PCR amplification assays for the detection of bacterial species in a variety of clinical or environmental samples and has frequently been used to identify various specific microorganisms because 16S rRNA genes show species-specific polymorphisms (Cloud, J. L., H. Neal, R. Rosenberry, C. Y. Turenne, M. Jama, D. R. Hillyard, and K. C. Carroll. 2002. J. Clin. Microbiol. 40:400-406). However, pure culture of bacteria are required and after PCR amplification the sample still has to be sequenced or hybridized to a micro-array type device to determine the species (Fukushima M, Kakinuma K, Hayashi H, Nagai H, Ito K; Kawaguchi R. J Clin Microbiol. 2003 June; 41(6):2605-15). Such methods are expensive, time consuming and labour intensive.
The present inventors have developed new methods for detecting microorganisms which can be adapted to general detection or initial screening assays for any microbial species.